The capability of crude antigen extracts from herpes simplex virus type 1-infected human diploid cells (AM) to induce protection against intraperitoneal challenge with homotypic but heterologous virus in mice was investigated. The administration of repeated doses of AM without adjuvant failed to confer significant protection. But one relatively small dose of AM with complete Freund's adjuvant resulted in a protective effect. A good correlation was observed between the protective activity of AM preparations and the content of neutralizing antigens as determined by the chromium release-inhibition test. Protection was observed even in animals which were free of neutralizing antibody at the time of challenge. The administration of one dose of cyclophosphamide (120 mg/kg body weight) on day 7 or day 14 after immunization depressed antibody formation but had no effect on the rate of protection. These findings seem to corroborate previous observations on the important role of cell-mediated immunity in protection against herpes simplex.